Many mechanical parts in automobiles and airplanes have special mineral coatings such as metal or ceramic for special properties such as hardness, wear resistance, etc. Such coatings are provided on parts such as gears, pulleys, shafts, and the like, made of metal. However, the metal part itself is often just a carrier for the coating and could be replaced by lighter weight, often easier to fabricate, polymer or polymer composite, if it were possible to suitably coat the plastic.
A simple technique for coating surfaces with metal or ceramic is by thermal spraying, also known as flame spraying, employing either powder or wire as a spray material. When attempting to thermal spray onto plastic, however, special problems are encountered. Upon cooling, the sprayed metal contracts and may warp or distort the plastic. The coating sometimes fails to adhere uniformly. The plastic substrate may melt from the material being sprayed and lose its shape, or the plastic surface may burn or decompose. Further difficulties are encountered with bonding to composite substrates such as polyimide bonded carbon fiber.
As disclosed in U.S. Pat. No. 4,388,373 (Longo et al) it has been found that plastic substrates can be flame sprayed with a mineral powder which has been admixed with small amounts of nylon and epoxy polymers in powder form. The powder particles in finely sub-divided form may be agglomerated with a binder or adhesive, mixed and dried, the agglomerates being composed of sub-articles of the individual components and being screened to recover particles of a particular size. The resulting agglomerates, or a simple powder mixture itself, can be flame sprayed in the conventional manner onto the substrate. The coating can range in thickness from about 25 .mu.m to 5 mm or greater.
A composite powder of austenitic stainless steel, epoxy and nylon according to the above-described patent (assigned to a predecessor of the present assignee) has been quite successful for producing a thermal spray coating on plastic substrates, either for bonding another thermal spray coating or for use as is. However, spray technique is somewhat critical causing variation in results, and further improvement in bonding and cohesive strengths has been in demand. Also, for certain applications a different plastic constituent for the coating material is necessary or desired, for example a high temperature plastic.
U.S. Pat. No. 3,723,165 (Longo and Durmann) discloses thermal spray coating materials comprising a high temperature plastic and a metal. In particular a silicon aluminum powder blended with poly(para-oxybenzoyl)ester in accordance with Example 1 of that patent has been highly successful commercially as an abradable coating for turbine blade seals and the like in gas turbine engines. Again, however, the spraying is technique dependent and improved bonding and cohesiveness are desired.
Various binders have been used or suggested for forming composite thermal spray powders. For example, U.S. Pat. No. 3,617,358 (Dittrich) discloses spray drying to produce thermal spray powders of fine particles agglomerated with any of a variety of binders. Usually the binder is burned off, but may not be in certain cases of an inorganic binder. For example, U.S. Pat. No. 4,593,007 (Novinski) teaches silicon dioxide derived from ethyl silicate in the binder for producing an abradable and erosion resistant coating of an oxide and aluminum.
Coupling agents, typically silane coupling agents, have been used traditionally in the fiber glass industry to improve the integrity and moisture resistance of composites reinforced with glass fibers. Organofunctional silanes are hybrid organic-inorganic compounds that are used as coupling agents. There exists more than one theory as to how such agents couple polymers and minerals, one of which is the formation of covalent bonds. The covalent bonds are formed during the curing cycle of the resin during the manufacture of the composite.
Additive agents also have been used in the formation of composite thermal spray materials. For example the above-mentioned U.S. Pat. No. 3,617,358 discloses various additives to aid in deflocculating, wetting and the like for producing the organically bonded agglomerates. U.S. Pat. No. 4,076,883 teaches a thermal spray wire of mineral powder bonded with polymer, in which surface active resins are added for aiding in the bonding of particles in the polymer of the wire. In both of these patents the additives are disclosed for the purpose of aiding in the formation of the composite spray material with a polymer, there being no teaching of the additive having any effect on the ultimate thermal sprayed coating. In each case the organic binder ingredients including additives are generally intended to burn off in the thermal spray process.
Organo-zirconate coupling agents have become known recently for enhancement of adhesion between inorganic and organic components in resin matrix systems. Such a zirconate is described in a brochure "KEN-REACT.RTM. Zirconate Coupling Agent - NZ 39 Product Data Sheet", Kenrich Petrochemicals, Inc., Bayonne N.J., Mar. 9, 1989. Properties are given in an undated paper "The Usage of Organometallic Reagents as Catalysts and Adhesion Promoters in Reinforced Composites" by G. Sugerman and S. J. Monte of Kenrich Petrochemicals, Inc.